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arialabelledby="calendar"><title id="calendar" lang="en">calendar icon</title><g fill="currentColor"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></g></svg><span>2021年11月9日</span><meta property="datePublished" content="2021-11-09T13:07:13.000Z"></span><!----><span class="words-info" arialabel="字数🔠" isoriginal="false" pageview="false" color="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon word-icon" viewbox="0 0 1024 1024" arialabelledby="word"><title id="word" lang="en">word icon</title><g fill="currentColor"><path d="M518.217 432.64V73.143A73.143 73.143 0 01603.43 1.097a512 512 0 01419.474 419.474 73.143 73.143 0 01-72.046 85.212H591.36a73.143 73.143 0 01-73.143-73.143z"></path><path d="M493.714 566.857h340.297a73.143 73.143 0 0173.143 85.577A457.143 457.143 0 11371.566 117.76a73.143 73.143 0 0185.577 73.143v339.383a36.571 36.571 0 0036.571 36.571z"></path></g></svg><span>约 1420 字</span><meta property="wordCount" content="1420"></span></div><hr></div><div class="toc-place-holder"><aside id="toc-list"><div class="toc-header">此页内容</div><div class="toc-wrapper"><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#回顾一下类加载过程" class="router-link-active router-link-exact-active toc-link level2">回顾一下类加载过程</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#类加载器总结" class="router-link-active router-link-exact-active toc-link level2">类加载器总结</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#双亲委派模型" class="router-link-active router-link-exact-active toc-link level2">双亲委派模型</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#双亲委派模型介绍" class="router-link-active router-link-exact-active toc-link level3">双亲委派模型介绍</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#双亲委派模型实现源码分析" class="router-link-active router-link-exact-active toc-link level3">双亲委派模型实现源码分析</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#双亲委派模型的好处" class="router-link-active router-link-exact-active toc-link level3">双亲委派模型的好处</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#如果我们不想用双亲委派模型怎么办" class="router-link-active router-link-exact-active toc-link level3">如果我们不想用双亲委派模型怎么办？</a></li><!----><!--]--></ul><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#自定义类加载器" class="router-link-active router-link-exact-active toc-link level2">自定义类加载器</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/jvm/classloader.html#推荐阅读" class="router-link-active router-link-exact-active toc-link level2">推荐阅读</a></li><!----><!--]--></ul></div></aside></div><!----><div class="theme-hope-content"><!--[--><h1 id="类加载器详解" tabindex="-1"><a class="header-anchor" href="#类加载器详解" aria-hidden="true">#</a> 类加载器详解</h1><h2 id="回顾一下类加载过程" tabindex="-1"><a class="header-anchor" href="#回顾一下类加载过程" aria-hidden="true">#</a> 回顾一下类加载过程</h2><p>类加载过程：<strong>加载-&gt;连接-&gt;初始化</strong>。连接过程又可分为三步：<strong>验证-&gt;准备-&gt;解析</strong>。</p><p><img src="https://my-blog-to-use.oss-cn-beijing.aliyuncs.com/2019-6/类加载过程.png" alt="类加载过程" loading="lazy"></p><p>一个非数组类的加载阶段（加载阶段获取类的二进制字节流的动作）是可控性最强的阶段，这一步我们可以去自定义类加载器去控制字节流的获取方式（重写一个类加载器的 <code>loadClass()</code> 方法）。数组类型不通过类加载器创建，它由 Java 虚拟机直接创建。</p><p>所有的类都由类加载器加载，加载的作用就是将 <code>.class</code>文件加载到内存。</p><h2 id="类加载器总结" tabindex="-1"><a class="header-anchor" href="#类加载器总结" aria-hidden="true">#</a> 类加载器总结</h2><p>JVM 中内置了三个重要的 ClassLoader，除了 BootstrapClassLoader 其他类加载器均由 Java 实现且全部继承自<code>java.lang.ClassLoader</code>：</p><ol><li><strong>BootstrapClassLoader(启动类加载器)</strong> ：最顶层的加载类，由 C++实现，负责加载 <code>%JAVA_HOME%/lib</code>目录下的 jar 包和类或者被 <code>-Xbootclasspath</code>参数指定的路径中的所有类。</li><li><strong>ExtensionClassLoader(扩展类加载器)</strong> ：主要负责加载 <code>%JRE_HOME%/lib/ext</code> 目录下的 jar 包和类，或被 <code>java.ext.dirs</code> 系统变量所指定的路径下的 jar 包。</li><li><strong>AppClassLoader(应用程序类加载器)</strong> ：面向我们用户的加载器，负责加载当前应用 classpath 下的所有 jar 包和类。</li></ol><h2 id="双亲委派模型" tabindex="-1"><a class="header-anchor" href="#双亲委派模型" aria-hidden="true">#</a> 双亲委派模型</h2><h3 id="双亲委派模型介绍" tabindex="-1"><a class="header-anchor" href="#双亲委派模型介绍" aria-hidden="true">#</a> 双亲委派模型介绍</h3><p>每一个类都有一个对应它的类加载器。系统中的 ClassLoader 在协同工作的时候会默认使用 <strong>双亲委派模型</strong> 。即在类加载的时候，系统会首先判断当前类是否被加载过。已经被加载的类会直接返回，否则才会尝试加载。加载的时候，首先会把该请求委派给父类加载器的 <code>loadClass()</code> 处理，因此所有的请求最终都应该传送到顶层的启动类加载器 <code>BootstrapClassLoader</code> 中。当父类加载器无法处理时，才由自己来处理。当父类加载器为 null 时，会使用启动类加载器 <code>BootstrapClassLoader</code> 作为父类加载器。</p><p><img src="https://my-blog-to-use.oss-cn-beijing.aliyuncs.com/2019-6/classloader_WPS图片.png" alt="ClassLoader" loading="lazy"></p><p>每个类加载都有一个父类加载器，我们通过下面的程序来验证。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ClassLoaderDemo</span> <span class="token punctuation">{</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;ClassLodarDemo&#39;s ClassLoader is &quot;</span> <span class="token operator">+</span> <span class="token class-name">ClassLoaderDemo</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;The Parent of ClassLodarDemo&#39;s ClassLoader is &quot;</span> <span class="token operator">+</span> <span class="token class-name">ClassLoaderDemo</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getParent</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;The GrandParent of ClassLodarDemo&#39;s ClassLoader is &quot;</span> <span class="token operator">+</span> <span class="token class-name">ClassLoaderDemo</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getParent</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getParent</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>Output</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>ClassLodarDemo&#39;s ClassLoader is sun.misc.Launcher$AppClassLoader@18b4aac2
The Parent of ClassLodarDemo&#39;s ClassLoader is sun.misc.Launcher$ExtClassLoader@1b6d3586
The GrandParent of ClassLodarDemo&#39;s ClassLoader is null
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><code>AppClassLoader</code>的父类加载器为<code>ExtClassLoader</code>， <code>ExtClassLoader</code>的父类加载器为 null，<strong>null 并不代表<code>ExtClassLoader</code>没有父类加载器，而是 <code>BootstrapClassLoader</code></strong> 。</p><p>其实这个双亲翻译的容易让别人误解，我们一般理解的双亲都是父母，这里的双亲更多地表达的是“父母这一辈”的人而已，并不是说真的有一个 Mother ClassLoader 和一个 Father ClassLoader 。另外，类加载器之间的“父子”关系也不是通过继承来体现的，是由“优先级”来决定。官方 API 文档对这部分的描述如下:</p><blockquote><p>The Java platform uses a delegation model for loading classes. <strong>The basic idea is that every class loader has a &quot;parent&quot; class loader.</strong> When loading a class, a class loader first &quot;delegates&quot; the search for the class to its parent class loader before attempting to find the class itself.</p></blockquote><h3 id="双亲委派模型实现源码分析" tabindex="-1"><a class="header-anchor" href="#双亲委派模型实现源码分析" aria-hidden="true">#</a> 双亲委派模型实现源码分析</h3><p>双亲委派模型的实现代码非常简单，逻辑非常清晰，都集中在 <code>java.lang.ClassLoader</code> 的 <code>loadClass()</code> 中，相关代码如下所示。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">ClassLoader</span> parent<span class="token punctuation">;</span>
<span class="token keyword">protected</span> <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> <span class="token function">loadClass</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">,</span> <span class="token keyword">boolean</span> resolve<span class="token punctuation">)</span>
        <span class="token keyword">throws</span> <span class="token class-name">ClassNotFoundException</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token function">getClassLoadingLock</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token comment">// 首先，检查请求的类是否已经被加载过</span>
            <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> c <span class="token operator">=</span> <span class="token function">findLoadedClass</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>c <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token keyword">long</span> t0 <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token keyword">try</span> <span class="token punctuation">{</span>
                    <span class="token keyword">if</span> <span class="token punctuation">(</span>parent <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token comment">//父加载器不为空，调用父加载器loadClass()方法处理</span>
                        c <span class="token operator">=</span> parent<span class="token punctuation">.</span><span class="token function">loadClass</span><span class="token punctuation">(</span>name<span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span><span class="token comment">//父加载器为空，使用启动类加载器 BootstrapClassLoader 加载</span>
                        c <span class="token operator">=</span> <span class="token function">findBootstrapClassOrNull</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token punctuation">}</span>
                <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">ClassNotFoundException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                   <span class="token comment">//抛出异常说明父类加载器无法完成加载请求</span>
                <span class="token punctuation">}</span>

                <span class="token keyword">if</span> <span class="token punctuation">(</span>c <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                    <span class="token keyword">long</span> t1 <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token comment">//自己尝试加载</span>
                    c <span class="token operator">=</span> <span class="token function">findClass</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>

                    <span class="token comment">// this is the defining class loader; record the stats</span>
                    <span class="token class-name"><span class="token namespace">sun<span class="token punctuation">.</span>misc<span class="token punctuation">.</span></span>PerfCounter</span><span class="token punctuation">.</span><span class="token function">getParentDelegationTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">addTime</span><span class="token punctuation">(</span>t1 <span class="token operator">-</span> t0<span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token class-name"><span class="token namespace">sun<span class="token punctuation">.</span>misc<span class="token punctuation">.</span></span>PerfCounter</span><span class="token punctuation">.</span><span class="token function">getFindClassTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">addElapsedTimeFrom</span><span class="token punctuation">(</span>t1<span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token class-name"><span class="token namespace">sun<span class="token punctuation">.</span>misc<span class="token punctuation">.</span></span>PerfCounter</span><span class="token punctuation">.</span><span class="token function">getFindClasses</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">increment</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token punctuation">}</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>resolve<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token function">resolveClass</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">return</span> c<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br></div></div><h3 id="双亲委派模型的好处" tabindex="-1"><a class="header-anchor" href="#双亲委派模型的好处" aria-hidden="true">#</a> 双亲委派模型的好处</h3><p>双亲委派模型保证了 Java 程序的稳定运行，可以避免类的重复加载（JVM 区分不同类的方式不仅仅根据类名，相同的类文件被不同的类加载器加载产生的是两个不同的类），也保证了 Java 的核心 API 不被篡改。如果没有使用双亲委派模型，而是每个类加载器加载自己的话就会出现一些问题，比如我们编写一个称为 <code>java.lang.Object</code> 类的话，那么程序运行的时候，系统就会出现多个不同的 <code>Object</code> 类。</p><h3 id="如果我们不想用双亲委派模型怎么办" tabindex="-1"><a class="header-anchor" href="#如果我们不想用双亲委派模型怎么办" aria-hidden="true">#</a> 如果我们不想用双亲委派模型怎么办？</h3><p><s>为了避免双亲委托机制，我们可以自己定义一个类加载器，然后重写 <code>loadClass()</code> 即可。</s></p><p><strong>🐛 修正（参见：<a href="https://github.com/Snailclimb/JavaGuide/issues/871" target="_blank" rel="noopener noreferrer">issue871<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a> ）</strong> ：自定义加载器的话，需要继承 <code>ClassLoader</code> 。如果我们不想打破双亲委派模型，就重写 <code>ClassLoader</code> 类中的 <code>findClass()</code> 方法即可，无法被父类加载器加载的类最终会通过这个方法被加载。但是，如果想打破双亲委派模型则需要重写 <code>loadClass()</code> 方法</p><h2 id="自定义类加载器" tabindex="-1"><a class="header-anchor" href="#自定义类加载器" aria-hidden="true">#</a> 自定义类加载器</h2><p>除了 <code>BootstrapClassLoader</code> 其他类加载器均由 Java 实现且全部继承自<code>java.lang.ClassLoader</code>。如果我们要自定义自己的类加载器，很明显需要继承 <code>ClassLoader</code>。</p><h2 id="推荐阅读" tabindex="-1"><a class="header-anchor" href="#推荐阅读" aria-hidden="true">#</a> 推荐阅读</h2><ul><li><a href="https://blog.csdn.net/xyang81/article/details/7292380" target="_blank" rel="noopener noreferrer">https://blog.csdn.net/xyang81/article/details/7292380<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new 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